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Location:
Terrigal
Registered:
May 2002
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Help with offset question
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Fri, 12 March 2004 01:04
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Hello, I know this question has been discussed before and there are references to it on the net, but i'm still struggling to work it out.
If I have a rear axle hub face to hub face that is 1310mm wide.
Now if I put on 7" (175mm) wheels with a +35mm offset.
Would that give me a track width of 1410mm or there abouts?
I got that figure from.. 175mm / 2 = 87.5mm. A positive offset of +35mm would make the mounting face at 52.5mm from the center of the wheel? So add 52.5mm to each side and ended up with 1414mm or there abouts?
Is that correct? Or am I way off?
Thanks
Joel
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Location:
Brisbane
Registered:
May 2002
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Re: Help with offset question
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Fri, 12 March 2004 01:16
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Track is measured from the centre of the tyre tread, so the width of your wheels is not relevant. Apart from that it all looks good. 
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Location:
Adelaide
Registered:
September 2003
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Re: Help with offset question
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Fri, 12 March 2004 02:17
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Norb's is correct on the track issue, but from my calcs, the numbers are as follows ;
Outer edge of the rims gives you a total width of 1435mm, with a theoretical track of 1260mm.
A +35 offset on a 7" rim means that you have a backspace on the rim of 112.5mm and a front space of 62.5 mm. Therefore overall width is 1310 + 62.5 + 62.5 = 1435mm.
Track is then the 1435mm - 62.5 - 62.5 (half width of each rim as per Norbs) = 1260 mm.
Cheers
Michael B
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Registered:
November 2002
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Re: Help with offset question
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Fri, 12 March 2004 06:30
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Whoa !
Fella's....
I think everyone is a little bit off here !
Let's start with a little sanity. A 7 inch wide rim will actually measure closer to 8 inchs in the 'flesh' when you include the outer rim 'lips' that hold the tyre on it.
And anyway, you don't need to work out backspacing or anything here, until you need to work out your clearances to the brakes and suspension, and whether the rim sticks out beyond the edge of the bodywork.
The offset is the measurement of the difference between the mounting face and the centre of the wheel. Joel, I'm not sure how you got those numbers mate !
Just make sure we are all taking the same way with offset.
Positive offset is the hub mounting face towards the outside of the car, negative moves the face towards the brakes on the car.
I think negative = brakes in my head to get this right, brakes to slow you down - go negative etc (um, ah - forget it !)
Anyway, if the hub to hub is 1310mm and the rims are +35 offset, the track will be 1240mm. (1310-35-35) The positive offset of the rims has moved the centre of each rim towards the middle of the car, and then this 'reduces' the track dimension.
I always relate this to my std RA23 which (on the rear) has a hub to hub of 1374mm, a listed track spec of 1310mm and the offset of 32 (mm) stamped on each steel rim just after the end of the rim size and safety markings. 14 x 4.5 JJ 32 (or whatever).
Joel, if you are talking RA23/28 axles, what I said for my RA23 would apply to your 28. If you are using a TA22 axle, I am fairly sure the original 13 inch steel rims are 22mm positive offset, the track is 1280mm and the hub to hub is 1324mm. Corrections invited please.
Cheers,
Mitch.
[Updated on: Sun, 14 March 2004 23:22]
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Location:
Terrigal
Registered:
May 2002
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Re: Help with offset question
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Mon, 03 May 2004 00:16
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Sorry to dig up an old thread here but this weekend I took some measurements to try and determine the exact offset of my current wheels.
With the new diff combo they do not fit anymore and rub the lips of the guards on the rear.
So they are 7" wheels, and from the rear of the wheel to the mounting face it is 120mm.
So the wheels is 175mm wide (7" x 25).
Middle would be 87.5mm. So from the middle of the wheel to the mounting face is 120mm - 87.5mm = 32.5mm
So is that correct, +32.5mm offset?
If that is correct, kinda sux as to move the wheel in even more will be like +50mm offset.
Can someone confirm my calculations? Is it odd for a RWD RA28 to have +32mm mags? I thought that was more a newer car/fwd offset?
Thanks
Joel
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Registered:
November 2002
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Re: Help with offset question
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Mon, 03 May 2004 00:46
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Joel,
I will start for the end of your e-mail mate.
Its not odd to have 32mm offset on a RA28. The standard
steel rims on a 23/28 are 32mm offset. As you fit wider wheels you have to reduce the offset downwards from this figure so that your track does not change too much and your wheels still fit inside the guards.
Without stripping a tyre off a rim, generally speaking an
alloy rim is usually 1 inch wider than the stated figure, as
this includes the rim lips that 'lock' the tyre onto the rim,
so your 7 inch rims (178mm) will actually measure closer to 8 inchs wide (203mm), than 7.
So it wont be 175mm from front edge to back edge. This is where you are getting confused.
Now the centre point of this rim is something like 102mm.
You measured the rear of the rim to the mounting surface (called the 'backspace' BTW) at 120mm, therefore your offset is going to be 120 - 102mm, 18mm. Let's say it is 20mm as most aftermarket wheels these days seem to be made with an offset in round figures, say 10mm or 40mm for example.
For this rim to fit properly the offset has to be less than this 20mm, say 10mm.
You make it hard for yourself with a new diff combo.
Can you tell me what this is again so I can help further ?
I am pretty sure that many aftermarket 7 inch rims come in offsets of 10mm.
Using the standard diff on my RA23 I have swapped the 5.5 x 14 inch steel rims for 6 x 15 Soarer rims. The offset change is from 32mm to 20mm, meaning that I have increased the track by 12mm each side, a total of 24mm, but under our state legal limit of 25mm. These rims are still well and truly under the guards on the rear too, which is a bit sad 
Cheers,
Mitch.
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Location:
Terrigal
Registered:
May 2002
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Re: Help with offset question
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Mon, 03 May 2004 02:08
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Hi Mitch, thanks for the reply.
That all makes more sense now.
The rear diff is from a corona mkII F series, which is around 1410mm face to face.
One thing I don't understand though.. Assuming my current wheels are 20mm offset, now I want to move the wheels inwards towards the center of the car, to decrease the track (as the wheels rub the guards).
So wouldn't I then want an offset greater than 20mm? Lets say I want the wheels to come in another 15mm each side to clear the guards. Wouldn't that then be an offset of 35mm?
Thanks mate
Joel
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Registered:
November 2002
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Re: Help with offset question
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Mon, 03 May 2004 02:17
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Joel,
No worries mate.
You have to reverse your thinking here. Remember that 'offset' is the difference between the wheel mounting face and the centre-line of the wheel itself. I'm not going to open up an 'argument' about the definition of negative and positive offset here, as this still confuses almost everyone, including me ! I just talk about offset as a whole.
Suffice to say, to pull the wheel inwards, you need to reduce the offset, not increase it, so you want to come down from 20mm to say 10mm or 5mm. Don't go back to the standard wheels offset of 32 - 35 mm. Reducing the offset moves the wheel inwards and gets you back towards the original track your car first had.
For our Celica's the general rule of thumb is that if you increase the wheel width you must decrease the offset.
Now, be careful at this point as the wheel moves in that you don't foul any rear disk calipers you have fitted or the rear shock absorbers either.
Cheers,
Mitch.
[Updated on: Mon, 03 May 2004 02:18]
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I supported Toymods
Location:
sydney.au
Registered:
August 2002
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Re: Help with offset question
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Mon, 03 May 2004 02:30
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do what i did, put skinny tyres on fat wheels
give your that little extra clearance you may need
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Location:
Terrigal
Registered:
May 2002
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Re: Help with offset question
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Mon, 03 May 2004 02:31
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Hmm where am I going wrong here, don't get it...
The offset is the distance between the centre line of the wheel and the backspace, in my case currently somewhere around 20mm...
Now if that figure became say 40mm, that means the distance between the centreline and backspace is larger, eg. increased another 20mm.
My understanding is if the figure between the center line and backspace is larger, the wheel will come inwards more?
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Registered:
November 2002
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Re: Help with offset question
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Mon, 03 May 2004 06:32
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Joel,
Couple of things here mate.
I don't recommend *skinny* tyres on a 'fat' rim, stick within the tyre makers guidelines for what size tyre fits what size rims.
Offset is the distance between the centre line of the wheel and the hub mounting face, not the backspace. You need to get backspace out of your mind completely, even though you measure it to get the offset dimension.
And, I owe you 1000 apologies, I have got it backwards, as I was thinking about wider rims on a standard size axle, not rims on a wider axle. Jeeze, sorry mate, I can see why you are confused !
OK, so now, when you increase the offset by say 20mm to 40mm, you are moving the hub mounting face (up) away from the centre line of the wheel, and moving away means moving the rim inwards, towards the diff.
Trying drawing a cross section sketch at a relative scale to see what it looks like from behind the car (say). And always remember that your RA28 track is 1310mm, the offset is 32mm and the hub to hub is 1374mm.
If you are using any wider axle, say hub to hub of 1410mm, that is an increase of 36mm over standard (18mm each side). To get back close to the original track of 1310mm, you will needs rims with an offset of 50mm. 32mm plus 18mm.
So, you are right mate, you're not going crazy !
If you can only get 7 inch rims with a 40mm offset you will end up increasing your track by a total of 20mm (10mm x 2) but that is still inside the legal limit.
Now I just wish I can edit my earlier posts a swap a few words around.
Mitch. 
[Updated on: Mon, 03 May 2004 06:34]
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Location:
Terrigal
Registered:
May 2002
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Re: Help with offset question
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Mon, 03 May 2004 06:42
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Haha thats cool dude it all makes sense now
Yeah a 10mm increase either side should be ok. So i'll see what style wheels I can turn up in a 40mm offset.
Like you say though, assuming these do not interfere with brakes/suspension mounting points etc.
Thanks for taking your time getting this all sorted and clear so I understand it!
Cheers
Joel
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Registered:
November 2002
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Re: Help with offset question
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Mon, 03 May 2004 06:51
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Just hit me with the stupid stick !
I was thinking about what happens to the track when you fit larger wheels on a standard axle at the same time as which way do you need to go if you want to 'fill the guards' ! Your problem was not either of these...... 
Anyway, yes, 40mm offset is the way to go, and last time I searched for wider rims to suit a RA23 I found quite a few that were 40mm offset so you should not have too many problems there.
And just so I have it clear in my head, if I want to fill the guards, I reduce the offset, if it rubs on the guards, I want to increase the offset.
Cheers mate,
Mitch.
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Location:
Terrigal
Registered:
May 2002
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Re: Help with offset question
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Mon, 03 May 2004 06:54
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I'll have to have a think about the front also.. But my head hurts a bit too much for that now haha.
But yeah, i've done the xt130 strut/hub combo with 4 spot brakes. This has increased the track slightly also. But i'd say using 40mm offset wheels would put it all in too much.
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Registered:
November 2002
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Re: Help with offset question
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Mon, 03 May 2004 07:03
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Lets leave this topic alone for today mate !
FWIW, I have also done the Hilux/Pug/Corona conversion on my car, and I have the Soarer 20mm offset rims on that, and they fill the front guards nicely, so just an idea, maybe you can buy aftermarket rims 40mm offset for the rear and 20mm offset for the front, but same design and get a near perfect soluton ?
Just food for thought !
Cheers,
Mitch.
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